Warner Bros. is attempting to enforce an embargo on “Watchmen” reviews, which have been popping up online fasting than crocuses after a spring rain. At least two of our sister News Corp. papers have published early notices following Monday night’s London premiere. Kevin Maher of The Times carefully straddles the fence, calling it “a mesmerising and brutalising experience, and will be, for some at least, more than worth the wait.” Robin Collin at the News of the World doesn’t mince words, saying it “isn’t the worst superhero movie ever made, mind. But it IS one of the most spirit-crushingly disappointing. Because this time round, it was supposed to be all so different. We were promised darkness. We were promised maturity. But what we’ve got, is 163 minutes of tin-ear dialogue and absurd violence, capped with one of the most trouser-wiltingly laughable sex scenes since…’Showgirls.’ ” Kyle Smith will have The Post’s review next week.